The Armenian-Azerbaijani War was a multi-sided conflict principally involving the First Republic of Armenia and the Azerbaijan Democratic Republic which occurred from March 1918 to November 1920, concurrent with the Russian Civil War.
During the chaos of the Russian Revolution and World War I, ethnic and religious tensions between the Armenian and Azeri communities of Baku - which had previously led to violent clashes in 1905 - were exacerbated. In March 1918, militias loyal to the pro-Turkish Azeri Musavat Party and the Armenian community in Baku engaged in armed confrontations which resulted in heavy casualties. Following the 1918 Treaty of Brest-Litovsk, during which the Russian SFSR ceded parts of the Caucasus to the Ottoman Empire, the Russian government in Trancaucasia declared the formation of the Transcaucasian Democratic Federative Republic on 22 April 1918 to prevent an Ottoman invasion. This short-lived union dissolved in May 1918 as a renewed Ottoman offensive loomed and the diverging goals of Armenians, Azeris, and Georgians jeopardized the state's existence; the Democratic Republic of Georgia declared its independence on 26 May 1918, followed by Armenia and Azerbaijan two days later.
Meanwhile, on 13 April 1918, the Armenian Revolutionary Federation-backed Bolsheviks created the Baku Commune in the aftermath of the March Days violence, which had left 12,000 Muslims dead. The Bolsheviks were involved with heavy fighting against the Ottoman Army in the Nagorno-Karabakh region, while Enver Pasha's Islamic Army of the Caucasus continued to advance northward. On 26 July 1918, the Armenian Revolutionary Federation ("Dashnaks"), Socialist Revolutionary Party, and Mensheviks overthrew the Bolshevik regime in Baku, and they created the Centrocaspian Dictatorship, which appealed for British help to stop the advancing Army of Islam's offensive into the Caucasus. On 14 September 1918, however, after six weeks of occupation, the British Army force in Baku was withdrawn to Iran, with most of the Armenian population escaping with the British forces. The Ottoman Army of Islam and its Azeri allies massacred as many as 20,000 Armenians in retaliation for the anti-Muslim violence that March, and the capital of Azerbaijan was moved from Ganja to Baku. On 30 October 1918, however, Ottoman Turkey made peace with the Allies, leading to the British once again occupying Baku. On 17 November 1918, 1,000 Commonwealth soldiers arrived in Baku, where martial law was implemented.
At the same time, an independent Armenia fought against the Azerbaijanis and Ottomans to expand and defend its borders, and ethnic violence became commonplace. In December 1918, the Azeris of Nakhchivan formed the Republic of Aras rather than accept a British border proposal which would assign Nakhchivan to Armenia, but, in June 1919, Armenian troops crushed the new republic. They soon came up against combined regular Azerbaijani and Ottoman troops, who reinstated Azerbaijani control over the city in July. On 10 August 1919, a ceasefire was signed as British troops withdrew from the Caucasus region.
Fighting resumed later that year, however, and fighting against the Armenians in Karabakh distracted the Azerbaijanis as the Soviet Red Army took advantage of labor unrest in Baku to invade Azerbaijan in April 1920. Armenian communists were emboldened to attempt a coup in Yerevan, but this coup attempt failed. In May 1920, the Soviets conquered the Karabakh region, and, in November 1920, another Soviet-backed communist uprising occurred in Armenia, justifying the Soviet general Anatoly Gekker's invasion of Armenia on 21 November 1920. Within a week, Armenia, too, had been conquered by the Soviets, with Yerevan falling on 4 December 1920. The Bolsheviks created the Azerbaijan SSR and Armenian SSR, and the USSR signed a treaty of friendship with the new Republic of Turkey later in 1921, with the Turks returning Gyumri to Armenia and Batumi to Georgia in exchange for Russian territorial concessions.